DNA methylation is essential for mammalian development and is a key epigenetic mechanism by which important developmental processes such as genomic imprinting and X chromosome inactivation are regulated. DNA methylation plays a crucial role in developmental regulation of gene expression, suppression of retrovirus infection and stable silencing of endogenous viruses. DNA methylation patterns are established during embryonic development and maintained in somatic tissues throughout adult life. Aberrant changes of DNA methylation patterns can lead to abnormal gene expression and diseases. In order to understand how DNA methylation patterns and thus gene expression profiles are regulated in normal development, it is important to study the enzymes that catalyze methylation and demethylation reactions. The identification of the Dnmt3 family of de novo methyltransferases opens many possibilities for experimental manipulation of this DNA modification system. This project will address a number of fundamental issues including the structure-function relationship of de novo methyltransferases Dnmt3a and 3b, the function of de novo methylation in cell proliferation, tissue differentiation, genomic imprinting and X inactivation, and the mechanism of transcriptional regulation of gene expression by DNA methylation. Genetic and biochemical approaches will be taken to investigate the functions of Dnmt3a and 3b in normal development and regulatory mechanisms of de novo methytransferase activities of Dnmt3a and 3b. Studies of Dnmt3a and 3b methyltransferases will provide new insights into epigenetic regulation of gene expression in development and will help understand how abnormal changes of DNA methylation patterns may cause diseases such as ICF syndrome and cancer.